In the prior art, partial pressure sensitive devices have been employed to analyze or discriminate among various residual gases on the basis of ionic charge-to-mas ratio (q/m). Such instruments as the helium leak detector and the quadrupole mass analyzer are examples of the operative technique. Certain other prior art apparatus such as ionization gauges and sputter ion pumps are known to exhibit pressure response to certain gases with marked increase or decrease of sensitivity in relation to particular gases. This principle has been used in prior art for leak detection e.g., Varian, Model No. 975-0010, has employed this technique in combination with a nulling circuit for the detection of small differences in pressure gauge output when a probe gas (to which the gauge is particularly sensitive or insensitive) enters the leak and thereby disturbs the equilibrium distribution of gases within the system.
Partial pressure gauges including leak detectors and residual gas analyzers which respond to q/m are subject to ambiguity in interpretation. For example, N.sub.2 and CO both exhibit integer molecular masses of 28 atomic mass units. He and .sup.2 H.sub.2 both possess mass 4 a.m.u. Differences occuring in higher decimal places can be distinguished only by a sufficiently high resolution instrument. Such higher resolution is nearly always accompanied by a proportionate decrease in the magnitude of the signal, thereby imposing severe limits for pressure sensitivity for a device of given resolving power. Moreover, achievement of adequate resolution from q/m analysis requires geometrical precision in fabrication and assembly of complex equipment.
The prior art also includes methods for partial pressure determination and leak detection based upon characteristic optical emission from selectively excited residual gases. Utilization of prior art apparatus for analysis of vacuum integrity requires a comparison to be made between the quiescent response of the instrument and the response of the instrument when a probe gas specific to the instrument is applied to a known or suspected leak. The presence of a small leak can only be known by the process of successfully probing for the leak. Consequently, a priori knowledge of the existence of a leak is limited by the precision with which the total pressure can be ascertained. The prior art, while providing means for detecting leaks by a systematic method, thus lacks provision for a priori indication of the presence of a leak.
Some prior art optical excitation pressure measurement apparatus teaches desirability of excitation with beams of electrons well defined in energy and direction in order to achieve selective excitation resulting in simple spectra. These limitations severely reduce the sensitivity of the apparatus. For an optical partial pressure gauge, signal strength is functionally related to the product N .times. I .times. .sigma.j(E) .times. l where N is the molecular density, I is the current density of ionizing electron particles, l is the path .times. length of the electron current, and .sigma.j(E) is the explicit energy dependent excitation cross section for a given quantum state of the molecule, atom, or ion represented in the molecular density. The quantity N is proportional to the pressure and .sigma.j(E) is a function expressing the probability for exciting state j with electrons of energy E which is in principal known or measurable for each state. In general, for a wide variety of states from the infrared to the ultraviolet spectral region, .sigma.j(E) rises rapidly to a maximum in the neightborhood of of a few volts to 150 ev and thereafter decreases slowly but never vanishes. The energy dependence of the excitation probability is controlled by the potential energy of the ionizing electrons. Some prior art teaches maintaining the electron energy at a fixed narrow value close to the maximum of the excitation curves for selected states of interest. When these selected states are characterized by excitation curves peaking at relatively low electron energy, other states with excitation curves peaking at higher energies contribute minimally to the resulting spectrum.